Powered snowthrowers of the type wherein the operator walks behind and guides the machine are well known and are generally either single stage or two stage. Two stage snowthrowers include two rotating snow handling elements, each of which is directly or indirectly coupled to the snowthrower's prime mover. The first stage typically includes a low speed, high torque rotating element which breaks up the snow and ice and augers it to the second stage element, a "fan." Rotating at higher speed than the auger, the fan propels the snow and ice upwardly through an adjustable discharge chute assembly.
A single stage snowthrower, by contrast, has only one snow handling element, an "impeller." Rotating about a horizontal axis, the impeller includes a plurality of paddles which extend radially outwardly from the impeller axis. The impeller is partially enclosed by a housing which is open in the front and has spaced side walls connected by a rear wall. The impeller spans between and is rotatably supported by the housing side walls. The lower portion of the housing rear wall is curved to conform to and closely shroud the impeller, whereas the upper portion of the housing rear wall is typically more or less planar, and extends upwardly to a set of guide vanes or a discharge chute. In operation, the impeller paddles draw the snow into the housing and against the arcuate lower portion of the rear wall. The snow continues up the rear wall upper portion and is ultimately discharged through the vanes or chute. The impeller of a single stage snowthrower must therefore single-handedly do the job of the auger and fan of a two stage machine, i.e., it must break up the snow, draw it into the housing, and propel it with considerable force up and out of the housing and away from the machine.
There are additional differences between typical single and two stage snowthrowers. For example, single stage machines are usually lighter, less powerful and less expensive than two stage machines, and commercially viable single stage machines, particularly ones with electric prime movers, must work within these constraints. This places additional demands on the single stage impeller. It must be inexpensive, light weight and efficient in terms of avoiding "losses" of various types which would waste the prime mover's limited power. Potential losses would include inertial, frictional (impeller to ground and impeller to snow), air resistance and snow slippage or leakage losses (slippage of the snow and ice between the outer tips of the impeller paddles and the arcuate lower portion of the housing rear wall). Impellers should also be capable of breaking up crusted snow and ice, to a degree, and work in such a way that they do not tend to clog the housing or stall or overload the prime mover under normal conditions.
Several different types of single stage impellers have been developed. One type of impeller includes a plurality, typically two, of flat paddles coupled to a horizontally rotating shaft or drum. The paddles typically include metallic (e.g., aluminum) inner flanges which hold flat plastic or rubber "blades." Such impellers are "closed" in the sense that the paddles are solid, without apertures sufficient to allow snow to pass through, from the centerline of the impeller to the outer radial edge thereof. This type of impeller is used on the gas engine powered S620 single stage snowthrower sold by The Toro Company, assignee herein. Such "closed," at least partially metal, impellers are fairly effective in terms of breaking up and throwing snow, but as a class they possess several characteristics which make them somewhat disadvantageous for some applications. For one thing, they are somewhat costly to produce inasmuch as they include aluminum or steel flanges or drums and require some assembly, i.e., attaching the rubber blades to the metal flanges. Another problem with such impellers is that they are fairly heavy, both from the standpoint of gross weight and rotational inertia. Thirdly, "closed" impellers encounter considerable air resistance because of the size of their paddles. Finally, the "closed" design of the flat paddles sometimes causes the prime mover to overload or stall in heavy, wet snow conditions since there are no internal passages within the impeller through which the snow can pass. Some flat paddle, closed impeller designs allow so much snow to slip around the outer tips of the blades that they avoid stalling the prime mover, but this is clearly disadvantageous from the standpoint of efficiency. "Closed" impellers also fail to produce much air flow. Applicants believe that a single stage impeller is preferably capable of pumping a considerable volume of air, and that this characteristic is particularly important in terms of throwing, or actually blowing, extremely dry, fluffy snow. Gas powered single stage snowthrowers having a flat bladed, metal flanged impeller can overcome some of the problems discussed above simply by brute force, and rubber bladed flat paddles are in fact quite useful in the sense that they help propel the machine along, making it virtually self-propelled in some cases. Electrically powered single stage machines having such impellers, however, suffer in their performance, as a class. Self propelling is not as important for the light weight electric models. Moreover, flat rubber paddles tend to flex under load and are somewhat difficult to fabricate and assemble. The flexing, wear and inherent impreciseness of flat rubber blades make it difficult to maintain a reasonably close fit between the impeller and the arcuate lower rear wall of the housing. This causes unnecessary leakage of snow around the outer tips of the "closed" paddles, and snow leakage of this type is particularly problematical for electric snowthrowers since they don't have the power to overcome such inefficiencies.
To address some of the problems associated with metallic/elastomeric flat paddled impellers, a curved paddle single stage impeller was developed. This impeller, shown in U.S. Pat. No. 4,694,594 which has been assigned to the assignee herein, has proven to have a particularly advantageous configuration when applied to gas-powered snowthrowers. Each paddle includes a central snowthrowing section which is curved forwardly from its midpoint to each side thereof to be concave, with the central section extending over at least about the middle 50% of the entire paddle's length. Two curved end sections are smoothly connected to the curved central section and are shaped to function as augers for moving a controlled volume of snow inwardly onto the central snowthrowing section. While this complex shape has been found to be very effective, this impeller is still made using the metal flange/elastomeric blade construction described above, and it is still a "closed" impeller. The metal flange/elastomeric blade construction makes it difficult to maintain precise dimensions for the curves, angles and housing fit, so efficiency is compromised to a degree. These characteristics render it less than ideal for electric snowthrowers, in particular.
In view of some of the problems discussed above in connection with metallic/elastomeric impellers, plastic impellers were developed. One type of plastic single stage impeller is represented by U.S. Pat. Nos. 3,452,460, issued to Cope et al., and 3,548,522, issued to Roper. These impellers have "closed" curved plastic paddles attached to a shaft or drum rotated by a snowthrower prime mover. While this type of impeller is lighter than prior metal flange/rubber blade impellers and might perhaps reduce some of the "losses" associated with closed, flat paddle, metal/rubber impellers, this type of impeller still possesses several disadvantages. A primary one is that such an impeller is still "closed," and can therefore cause or contribute to the stalling or overloading of the prime mover in heavy, wet snow conditions. Also, this type of impeller would still be fairly expensive to produce in that separate paddles have to be attached to a shaft, adding fastener cost and assembly time to the overall cost of the impeller; and the tolerance buildup of assembly requires more clearance to the housing, causing unnecessary snow slippage around the outer tips of the blades.
Another type of plastic impeller is represented by U.S. Pat. Nos. 4,295,285 and 4,325,195, issued to Stevens and Comer, respectively, and assigned to The Toro Company, assignee herein. The Stevens and Comer snowthrowers are small, electrically-powered "Power Shovel" devices. The impellers are only about 11 inches wide and about 4 inches in diameter. They are "open" impellets in that they have a substantially open center to help avoid prime mover overload and to pump more air to enhance the removal of light, fluffy snow. The impellets shown in these references are also similar in that they include flat paddles. Comer and Stevens differ, however, in the way that the impeller is supported and driven. In Comer, a full length shaft, coupled to the prime mover, supports the paddles. Torque is transmitted from one end of the impeller to the other through this shaft. By contrast, in the Stevens reference there is no through shaft. Torque or power is applied to one end of the impeller and is transmitted by the flat plastic blades across the entire length of the impeller. While the Comer and Stevens impellets address many of the problems associated with the prior art, primarily because of their "open" configuration, they possess certain shortcomings, as well, particularly when applied to snowthrowers larger than the Power Shovel type. For example, the blades of Comer and Stevens are flat. It has been found that curved impellers having a particular shape possess several advantages. Reference is again made to U.S. Pat. No. 4,694,594, issued to Thorud et al., and assigned to the present assignee. Although the open design flat blade rotor of Comer and Stevens does pump air and throw light fluffy snow out of the housing, wind quickly disperses it, whereas an open curved rotor pumps the light fluffy snow into a concentrated stream that can be directed where desired and is much less prone to dispersal by the wind. Also, Comer's impeller is not substantially made of one piece of plastic. The blades or vanes are attached to circular plates by means of right angle tabs or the like. Finally, neither of the impeller shaft configurations shown in Comer and Stevens is optimum for larger snowthrowers. The Comer impeller includes a through shaft to which is attached the blades of the impeller. This adds cost and unnecessary rotational inertia to the impeller. While Stevens does not include a through shaft, it also does not include anything to stabilize or prevent distortion of the impeller under load. While distortion of an impeller, which could cause snow leakage or slippage around the outer tips of the blades, is probably not much of a problem for a Power Shovel type (i.e., small, low power) snowthrower, impeller distortion could significantly diminish the performance of a larger, higher power machine. Moreover, the Stevens and Comer impellers include large circular "spiders" extending from the axis of the impeller to the blades, presumably to strengthen the impeller and to act as a ground bearing structure. The spiders are the same size and shape as the circular end plates which are adjacent to the housing side walls. The large, circular spiders potentially reduce air flow and certainly increase weight, rotational inertia, and frictional (impeller to ground and impeller to snow) losses.
Also, the Comer and Stevens impellers do not include means specifically designed for breaking up crusted snow and ice. In fact, fully round spiders will tend to slide over hard snow and prevent the blades from hitting the snow. Even if the flat impeller blades themselves can theoretically serve this function to a degree, the Power Shovel prime mover, a relatively small electric motor, is not particularly well suited for this purpose.
The present invention addresses the above-noted problems associated with prior art single stage snowthrower impellers. That is, the present invention is directed toward a single stage snowthrower impeller which is very efficient, very effective in throwing all types of snow, cost effective, applicable to mid to large single stage walk-behind snowthrowers, and, in preferred embodiments, can break up crusted snow and ice and propel the snowthrower to a degree. The impeller of the present invention is particularly well suited for electric single stage snowthrowers.